This invention relates primarily to an internal resistive system for television cathode ray tubes for protecting a tube and its associated circuitry from destructive electrical arcs and arc currents and for eliminating static charge accumulations inside the neck of a tube.
The envelope of a television cathode ray tube comprises a glass funnel and a mating faceplate. The funnel has a neck within which is located an electron gun. The faceplate of the tube has a fluorescent screen on which is impressed a very high DC voltage--typically in the range of 20-30 kilovolts or more. The screen is stimulated by one or more electron beams generated in the gun.
Conductive coatings on the inside and outside of the funnel serve as a large capacitor which filters the high voltage supplied to the screen. The inner conductive coating is at screen potential and also serves to transmit the screen voltage to the neck of the tube where it is applied to a high voltage anode electrode at the forward end of the electron gun.
The electron gun has one or more cathodes and a series of closely spaced electrodes which shape, accelerate and focus the electron beam(s) generated in the gun. To accomplish these functions, the various electrodes require widely different electrical potentials. The large voltage differences established between certain high voltage and low voltage electrodes in the gun creates a susceptibility to arcing between the electrodes, e.g., should there exist particulate foreign matter in an inter-electrode space, a burr on an electrode, a misaligned or improperly spaced electrode, or the like. Large voltage differences between the gun electrodes and other tube internal components also establish arc-conducive conditions. When the conditions for arcing exist, the high voltage filter capacitor, with its immense stored electrical energy, will within a few microseconds or less dump its stored charge.
Because the instantaneous peak arc currents can reach hundreds of amperes in magnitude, great destruction can be wrought by such arcs. External circuitry can be damaged by transient currents and voltages induced in the associated receiver circuitry. Internal gun parts can be eroded to the point of inoperability or severely reduced in their effectiveness. High arc currents are capable of sputtering electrode materials onto adjacent surfaces, resulting in the formation of electrical leakage paths. Further, arcing in a tube during its normal operation can result in a loud audible report which may be quite disturbing to a viewer.
In recent years, the design evolution of color picture tubes has taken a direction tending to exacerbate the arcing problem. The design for greater picture brightness has driven the screen voltages inexorably upward toward and even beyond 30 kilovolts. A trend toward wider beam deflection angles and a desire to minimize power consumption have dictated the use of tubes with smaller neck diameters. A small neck diameter implies a more closely confined environment for the electron gun, with the attendant increased probability of arcing between components of the electron gun assembly or between the gun assembly and the containing tube envelope.
In order to reduce tube arcing, it is routine today to design color tubes and electron gun assemblies with every effort to maximize intercomponent spacing, to minimize points of field concentration, and otherwise to configure the tube and gun structures to minimize the tendency of a tube to arc. After a tube receives its electron gun and the envelope is sealed, it is commonplace to "spot-knock" (high voltage condition) the gun. "Spot-knocking" is an operation wherein a pattern of fluctuating and constant voltages of high magnitude are applied to the tube to "knock" (remove) loose particles which may have lodged between gun electrodes, burrs on electrode parts, and other agents which might lead to arcing of the tube during its normal operation. Typically peak voltages during spot-knocking are much higher than the screen operating voltage. Spark gaps, diodes, filters, gas discharge lamps, decoupling circuits, and other protective devices are commonly provided in the associated receiver (at significant cost) to protect receiver circuitry from damage by arc-induced currents and voltages.
Television picture tube manufacturers have long attempted to develop an internal resistive element which would be coupled in series with the high voltage filter capacitor and the electron gun to suppress the magnitude of arc currents and thereby overcome the potentially destructive effects of arcing in the tube during tube operation.
The requirements for such an internal resistance element are, however, extremely severe. Following are some of the requirements, not necessarily in their order of importance, of an internal resistive element or system of elements for protecting a television picture tube against arcing.